Pallets are utilized in a wide variety of industrial processes for supporting and transporting materials from one place to another. Often the materials on the pallet also require processing such as heating or cooling. For example, in the production of iron pellets used as raw material to produce steel, balls of iron ore mixed with various elements are heated to a high temperature in a system that includes a traveling grate, rotary kiln, and an annular cooler. The pellets enter the system as soft, moist green balls and exit the system as hard, fully oxidized pellets that will not degrade during shipping and handling and that have an ideal size and shape for use in the steel making process. The hard pellets discharge from the rotary kiln at a temperature of approximately 2300° Fahrenheit and have to be cooled to a temperature of approximately 300° Fahrenheit. The hot pellets discharge from the rotary kiln and fall onto slotted pallets as they pass into the inlet area of the annular cooler, and are conveyed in a circle until cooled to a desired temperature. In the annular cooler, ambient air is forced up through the pellet bed supported by the pallets to cool the hot pellets. The rotating conveyor rotates at a speed generally between 1–2 revolutions per hour. The pallets travel in a circle around the annular cooler and each pallet typically comprises about a 10° segment of the circle. After having traveled nearly a complete circle around the cooler, the cooled pellets are discharged from the annular cooler in a dump zone.
The primary purposes of the pallet construction is to support the loading applied to the pallet deck and to maximize the amount of air flow through the pallet to cool the pellets. Under normal operating conditions, the loads applied to the pallet include the weight of the pellet bed and the impact loading of the pellets as they fall from the kiln discharge onto the pallet. An individual pellet is typically ⅜″ in diameter so the impact loading of an individual pellet is quite small. Under normal operation conditions where the discharge of the pellets from the kiln is free flowing, the pallet does not see a significant impact loading condition. The total number of pellets on a pallet at a bed depth of 33 inches weighs approximately 14 tons. Therefore, the pallet must have sufficient structural integrity to support this loading.
However, under upset conditions, large chunks of agglomerated pellets can form in the kiln. This condition will result in a large impact loading on the pallet deck as thousands of pellets agglomerated together fall from the kiln as one large mass. If such large chunks are not prevented from falling onto the pallet deck by grizzly bars or other means, then the pallet construction must support large impact loading.
As capacities of pelleting systems have increased, the cooling requirement of the annular coolers have also become greater. The capacity of existing systems has increased to the point that the annular coolers are often the bottleneck in the system, prohibiting further increase in system pelleting capacity. One of the limiting factors in increasing the cooling capacity of existing annular coolers is the available slot area in the existing pallet constructions available for cooling air to pass through. Therefore, further increase in slot open area is desired to increase system cooling efficiency and increase the capacity and throughput of the annular cooler. This increase in slot open area must, however, be achieved while retaining the operational and dimensional features of the existing pallet design. For example, the depth of the pallet support frame cannot be greater than the existing pallet design because the improved pallet must fit into and operate within existing cooler installations.
In a typical annular cooler, the various cooling zones are partitioned off into various sections for heat recovery purposes and there is a limited clearance into which the pallet constructions must pass. The small clearance allows for sealing between the different zones and further increases the efficiency of the annular cooler. Therefore, any improved pallet structure must be designed so that there is a section of the pallet construction that passes through the small clearance in such a way that an air seal is formed between the pallet and the cooler.
It is therefore desirable to provide an improved pallet construction which increases the amount of open area to increase the volume of cooling air that passes through the pallet. It is further desirable to provide such a pallet construction which maintains or increases the amount of load capacity provided by known pallet constructions. It is further desirable to provide such a pallet construction which has the same or greater structural strength as existing designs and which properly supports small-sized material, such as the above-mentioned iron pellets. It is further desirable to provide such a pallet construction which retains certain existing configurations of support members, such as depth thereof, such that the improved pallet fits into and operates properly with existing cooler installations.